1. Field of the Invention
The present invention relates to an object, a method, a manufacturing method, a process, a machine, manufacture, or a composition of matter. In particular, the present invention relates to, for example, a semiconductor layer, a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a driving method thereof, or a manufacturing method thereof. In particular, the present invention relates to, for example, a semiconductor device including a transistor, a display device including a transistor, or a light-emitting device including a transistor, or a driving method thereof. Alternatively, the present invention relates to, for example, an electronic device including the semiconductor device, the display device, or the light-emitting device.
Note that in this specification, a semiconductor device refers to any device that can function by utilizing semiconductor characteristics, and an electro-optical device, a semiconductor circuit, an electronic appliance, and the like are all semiconductor devices.
2. Background Art
A technique for forming a transistor by using a semiconductor film formed over a substrate having an insulating surface has attracted attention. The transistor has been widely used for semiconductor devices such as integrated circuits and display devices. A silicon film is known as a semiconductor film applicable to a transistor.
Whether an amorphous silicon film or a polycrystalline silicon film is used as a semiconductor film in a transistor depends on the purpose. For example, in the case of a transistor included in a large display device, an amorphous silicon film, which can be formed using an established technique for forming a film over a large substrate, is preferably used. On the other hand, in the case of a transistor included in a high-performance display device where driver circuits are formed over the same substrate, a polycrystalline silicon film, which can form a transistor having a high field-effect mobility, is preferably used. As a method for forming a polycrystalline silicon film, high-temperature heat treatment or a laser light treatment which is performed on an amorphous silicon film has been known.
In recent years, an oxide semiconductor film has attracted attention. For example, a transistor which includes an amorphous oxide semiconductor film containing indium, gallium, and zinc and having a carrier density lower than 1018/cm3 is disclosed (see Patent Document 1).
An oxide semiconductor film can be formed by a sputtering method or the like, and thus can be used for a channel formation region of a transistor in a large display device. A transistor including an oxide semiconductor film has high field-effect mobility; therefore, a high-performance display device where driver circuits are formed over the same substrate can be obtained. Moreover, there is an advantage that capital investment can be reduced because part of production equipment for a transistor including an amorphous silicon film can be retrofitted and utilized.